This invention relates to the field of power plant technology. It concerns a combination power plant with an injection device for injecting water into the live steam according to the preamble of claim 1. Such a combination power plant is known, for example, from applicant""s document DE-A1-195 45 668.
In combined gas/steam power plants or combination power plants in which the hot waste gases of one or more gas turbine(s) are used to generate steam in the water/steam cycle of one or more steam turbine(s), the steam temperature depends greatly on the load of the gas turbine(s) and the environmental conditions. In order to be able to flexibly adapt the steam temperature of the live steam to the requirements of the steam turbine(s), waterxe2x80x94if necessaryxe2x80x94is injected into the live steam, so that the temperature of the live steam may be reduced, more or less, as needed. The simplified schematic of such an (exemplary) combination power plant as currently used is shown in FIG. 1. The combination power plant 10 comprises a gas turbine system 11 (shown for simplicity""s sake as a function block), a steam turbine 13 with high pressure stage 14, medium pressure stage 15, and low pressure stage 16, a waste heat steam generator 25 with various evaporation and heat exchanger devices, a feed water container 18, and a condensation device 17 (also shown only as a function block). In the example, the gas turbine system 11 and the steam turbine 13 drive a common generator 12. The waste heat steam generator 25 of the example comprises three evaporators, i.e. a low pressure evaporator 27, a medium pressure evaporator 30, and a high pressure evaporator 34. Each of the three evaporators 27, 30, 34 adjoins a corresponding steam drum, i.e. a low pressure steam drum 28, a medium pressure steam drum 31, and a high pressure steam drum 35, from which the condensate in each case is pumped by means of recirculation pumps 40, 41, and 42 to the adjoining evaporator. Hot waste gases that are discharged from the gas turbine system 11 (arrows in FIG. 1) flow through the waste heat steam generator 25 from the bottom to the top. At the cold, upper end of the waste heat steam generator 25, a low pressure preheating stage (low pressure economizer) 26 is provided. Between the evaporators 27 and 30 is a medium pressure preheating stage (medium pressure economizer) 29. A high pressure preheating stage (high pressure economizer) 32 is located between evaporators 30 and 34. Also located between evaporators 30 and 34 is a medium pressure superheater 33. And finally, at the hot, lower end of the waste heat steam generator 25, a high pressure superheater 37 and an intermediate superheater (xe2x80x9creheaterxe2x80x9d) are provided. The internal construction of waste heat steam generator 25 is therefore similar to the one in U.S. Pat. No. 5,647,199. The function of the system according to FIG. 1 can be described as follows: Feed water is pumped from the feed water container 18 via one of two possible ways by a first feed water pump 19 through a first feed water line 21 to the waste heat steam generator 25, and is heated there consecutively in preheating stages 26, 29, and 32. The preheated feed water flows into the high pressure steam drum 35, and is evaporated in the connected high pressure evaporator 34. The resulting steam from high pressure steam drum 35 is superheated in the following superheater 37, and is fed as live steam via a live steam line 51 to the high pressure stage 14 of the steam turbine 13. After flowing through the high pressure stage 14, additional steam from the medium pressure steam drum 31 that has been superheated in the superheater 33 is added to this steam before it is reheated in the intermediate superheater 36, and it is then passed via a hot steam line 52 to the medium pressure stage 15. The medium pressure steam drum 31 is hereby supplied by a second feed water pump 20 via a second feed water line 22 with feed water that is preheated in the two preheating stages 26 and 29. The low pressure steam drum 28 is also supplied by the second feed water line 22 with feed water that is preheated in the first preheating stage 26. The steam from the low pressure steam drum 28 is supplied via the medium pressure steam line to the medium pressure stage 15 of the steam turbine 13. After the steam has consecutively passed through the medium pressure stage 15 and the low pressure stage 16, it is condensed in an condensation device 17, and the condensate pumped back into the feed water container 18. As previously mentioned, the temperature of the steam in the live steam line 51 and hot steam line 52 depends greatly on the discharge temperature of the gas turbine system 11. To make it possible that the steam temperature can be changed without intervening in the operation of the gas turbine system 11, injection devices 38 and 39 that can be used to inject water into the steam and thus to reduce the steam temperature have been provided in each of lines 51 and 52. The injection devices 38 and 39 receive their water via corresponding supply lines 23 and 24 from the feed water lines 21 and 22, in which the required injection pressure is already present due to the feed water pumps 19, 20.
During operation of the system, it may occur thatxe2x80x94in order to improve the start-up behaviorxe2x80x94the steam temperature is adjusted down in a controlled manner at certain time intervals by injecting water, while no water needs to be injected at other time intervals. This means that the injection devices 38, 39 are only required intermittently. This operation results in high loads due to alternating stresses within the injection devices 38, 39 installed in the hot steam stream. During injection operation they are brought to a temperature close to that of the injected water which, in order to have a sufficient initial pressure (see FIG. 1) is removed directly from the (approximately 60xc2x0 C. cool) feed water. In contrast, the injection devices 38, 39 are brought to steam temperature between the periods of the injection operation, whereby said steam temperature can easily be above 500xc2x0 C. and up to 600xc2x0 C. in modern systems. These thermal and thermo-mechanical alternating stresses can result in premature breakage of components of the injection devices. In principle, the situation could be somewhat improved if preheated water were removed for injection from the preheating stages 26, 29, 32 of the waste heat steam generator 25. But with this procedure, the water in supply lines 23, 24 also remains cold in most cases until it reaches injection devices 38, 39, since the lines are not located within the warm waste gas stream. This means that at the beginning of the injection process, first cold water flows through the injection devices 38, 39 and results in thermal stresses. Although the extent and duration of these stresses are limited, they are large enough to pose a serious risk of breakage.
It is therefore the objective of the invention to modify a combination power plant of the above-mentioned type in such a way that the described disadvantages are avoided, and, in particular, so that the thermal stresses in the injection device(s) are reduced to such an extent that breakage due to alternating stresses can be largely avoided. This objective is realized with the entirety of the characteristics of claim 1. The core of the invention is that the mechanical stresses induced by temperature differences are reduced to a harmless level by preheating the injection water or maintaining its temperature up to a short distance from the injection devices or injection nozzles. A first preferred embodiment of this combination power plant according to the invention is characterized in that the steam turbine comprises a high pressure stage and a medium pressure stage, that the live steam line is connected with the inlet of the high pressure stage, that the outlet of the high pressure stage is connected with an intermediate superheater located in the waste heat steam generator, said intermediate superheater reheating the steam from the high pressure stage and delivering it via a hot steam line to the medium pressure stage, that a second injection device for injecting water into the hot steam is located in the hot steam line, said injection device being supplied via a second supply line with injection water from a removal point in the water/steam cycle, and that in the second supply line of the second injection device second means for preheating the injection water are located a short distance before the injection device. This makes it possible to use the advantages of the invention also for systems with intermediate superheating. The preheating can be accomplished in a particularly effective manner if, according to another preferred embodiment of the invention, the first or second means each comprise a condenser that is connected via a tap line to a steam-carrying line of the water/steam cycle and through which the injection water flows. Such a leakage steam condenser has the advantage of maintaining the heat or preheating, which results in practically no losses. Only during the injection process does a slightly larger amount of steam flow as a result of the condensate that accrues in a higher amount. The preheating causes a higher injection amount, which, however, results in a higher performance of the steam turbine, so that this loss is substantially compensated for again. In place of the condenser, a gas/gas heat exchanger, water/gas heat exchanger, or electric heater can be used as a heating device within the framework of the invention. In principle, the condensers can be connected on the steam side to any of the steam-carrying lines. It is possible, but not mandatory, to place the connection on the same line. The pressure level determines the possible preheating temperature and thus the criteria for the selection of the adequate steam system.In an exemplary embodiment particularly advantageous the condenser of the first means is connected via its tap line to the live steam line or if the condenser of the second means is connected via its tap line to the hot steam line. In an exemplary embodiment, heat steam generator comprises a low pressure evaporator, medium pressure evaporator, and high pressure evaporator, which adjoin a low pressure steam drum, a medium pressure steam drum, and a high pressure steam drum, respectively, and the condensate of the first condenser is fed to the medium pressure steam drum or the condensate of the second condenser is fed to the low pressure steam drum, and the first or second condenser adjoins a condensomat in order to regulate the fluid level in the condenser. The preheating device is therefore practically self-regulating and permits a preheating temperature near the saturation temperature of the heating medium. The size of the leakage condenser and condensomat hereby depends on the maximum injection amount of the respective injection device. Further embodiments can be derived from the secondary claims.